The present invention relates to a method for preparing rubbery composite materials whereby a substrate such as metals, plastics, and ceramics and a rubber composition are bonded with excellent adherence onto a composite body.
Composite materials have been of great interest in a variety of industrial fields because of their superiority in function, reliability, and durability and in cost to the respective components per se. There have been developed a number of new composite materials based on a specific combination of various components.
Composite materials are generally classified into blend and laminate types depending on their composite form. Among those types a more active development work has been focused on the laminate type composite materials because they can be conferred with unique properties as exemplified by anisotropy. Inter alia, rubbery composite materials constitute one class of material where the development work is most active as they can be utilized in a wider variety of fields including not only automobile parts such as tires, vibration dampers, and bumpers, but also electric and electronic parts, and sports items.
Production of a laminate type composite material depends on whether or not a substrate such as fibers, metals, etc. and a rubber compound can be fully bonded. Particularly when the lamination is of a rubber composition, a high level of technology is required because of the special factor that rubber matrix repeats great dynamic deformation. An improvement in bonding techniques is one of main concerns of laminate type composite materials, inter alia, rubbery composite materials.
In the prior art, laminate type composite materials, inter alia, rubber composite materials are produced, for example, by an indirect bond method wherein an adhesive is applied to at least one of a substrate and a coating of rubber composition to bond them together with or without forming a finely roughened surface on at least one of them. Another bonding method is by forming a metal thin film of zinc or brass on a substrate by wet plating and bonding a vulcanizable rubber composition to the metal thin film under heat and pressure.
The former method, that is, adhesive bonding between a substrate and a coating with or without a roughened surface on the substrate has been commercially practiced in the manufacture of vibration damping rubbers or the like. However, there remain many problems including coating considerations such as adherent pretreatment and adhesive maintenance, complicated operation, and safety and hygienic problems associated with the use of organic solvent and the post-disposal of a pretreating agent. Problems also arise when the substrate is of a plastic material. Not only is the adhesive which can bond a plastic material into a composite material limited to a special class of adhesives, but also the bondable substrate is limited to fewer types of plastic materials such as nylon and ABS resins. This indirect bonding method cannot be essentially used in the manufacture of tires and similar articles which are exposed to an extremely severe environment during service. There is the need for an adhesive-free bonding technique in the industrial fields of tire manufacturers and the like.
The latter method, that is, composite material production utilizing a wet-plated metal thin film is only successful with a few metals used as a metal thin film for composite bonding, usually zinc, brass (Zn--Cu alloy), and bronze (Sn--Cu alloy). The rubber composition which can be laminated on the metal thin film is accordingly limited to a few types. In utilizing the wet plating method, control of the thickness of a metal thin film is inherently done. The film tends to be irregular in thickness as often observed in electrolytic plating. A metal thin film having a uniform thickness cannot be obtained unless the thickness exceeds several microns. The intrinsic nature of a metal thin film having such a substantial thickness cannot be ignored depending on the intended type of composite material and sometimes detracts from the flexibility of the resulting composite material. Disposal of spent solutions with a strong acid or alkali is also a problem.
More specifically, the prior art methods for adhesion of a metal surface and an unvulcanized rubber are as follows:
(i) a method comprising applying an adhesive containing a chlorinated polymer, cyclized rubber or the like to a cleaned metal surface to bond it to a rubber;
(ii) a method comprising laminating a rubber composition containing resorcin, hexamethylenetetramine or the like on a metal such as brass, bronze, zinc or the like, and vulcanizing the rubber composition, thereby bonding the metal to the rubber composition; and
(iii) a method comprising laminating a rubber composition containing an organic cobalt salt such as cobalt naphthenate, cobalt stearate or the like on a metal such as brass, bronze, zinc or the like and vulcanizing the rubber composition, thereby bonding the metal to the rubber composition.
Method (i), which is referred to as an "indirect process", is mainly employed on the production of rubber vibration insulator. On the other hand, methods (ii) and (iii), which are referred to as a "dry process" or "direct process", are employed in a more severe use condition including the production of tire, conveyer belt, hose, etc. Method (ii), however, has a problem in that, although it provides good adhesion of the rubber composition and metal in an initial stage, the adhesion property would deteriorate after a long period of usage especially under a severe condition, such as high temperature, high moisture and corrosion circumstances. Further, method (ii) is inferior in workability and productivity. Therefore, method (iii), which uses an organic cobalt salt, usually in an amount of 1 to 3 parts by weight per 100 parts by weight of a rubber component, is widely employed in the field of rubber articles, tires, etc. because the addition of the organic cobalt salt to a rubber composition would accelerate the adhesion reaction between the rubber composition and metal (brass, bronze and zinc), improve corrosion resistance and heat resistance of the adhesion interface, and stabilize the adhesion strength for a long period of time. See W. J. van Ooij, ACS Rubber Division 112th Meeting, No. 19, October 1977, Pages 18 to 19; W. J. van Ooij et al, ACS Rubber Division 124th Meeting, October 1983, Rubber Chemistry and Technology, 57, 686-702.
As described above, when bonding a metal (typically brass, bronze and zinc) to a rubber composition, it is a common practice in the field to blend an organic cobalt salt to the rubber composition. Such a rubber composition containing an organic cobalt salt is vulcanized by using sulfur. In this case, it is recognized that the sulfur content should be large, usually more than 6 parts by weight per 100 parts by weight of a rubber component (W. J. van Ooij et al, ACS Rubber Division 118th Meeting, No. 36, October 1980, Page 15). However, the addition of an organic cobalt salt will accelerate heat aging of rubber and deteriorate the rubber property. Further, as the sulfur content becomes larger, the heat aging property of rubber becomes worser.
Benko U.S. Pat. No. 4,517,066, Haemers U.S. Pat. No. 4,255,496, Palmer U.S. Pat. No. 2,640,523 and Van Ooij U.S. Pat. No. 4,218,517 all disclose bonding methods of a rubber composition and a substrate.